The present invention relates to a process and system for producing spherical particles of turbostratic or hexagonal boron nitride and to a spherical boron nitride precursor produced by such method.
Boron nitride xe2x80x9cBNxe2x80x9d is a chemically inert, refractory material which has excellent electrical insulating properties and high thermal conductivity. Accordingly, boron nitride is an ideal candidate as a filler material in organic polymers for use in the electronics industry particularly to encapsulate electrical components, integrated circuits and solid state devices. However, despite such beneficial properties the morphology of boron nitride, as presently manufactured, minimizes its utility for thermal management applications. Boron nitride powder is currently manufactured by high temperature reactions between simple inorganic raw materials and the powder has a hexagonal structure with a platelet morphology. The platelet morphology of boron nitride adversely impacts the volume loading of BN particles as well as the viscosity of the polymer composite during processing. This, in turn, limits its utility as a filler material particularly for thermal management applications. Stated otherwise, its platelet morphology inherently limits the volume ratio of filler to polymer necessary to form molding and underfill compounds possessing the electrical insulating and thermal conductivity properties necessary for thermal management applications in the electronic industry. It is however, anticipated that spherically shaped BN particles will pack more efficiently and form a more dense matrix composite in a polymeric material. This should, in turn, permit achievement of the loading volumes at favorable polymer composite processing viscosities required to form desirable molding, encapsulant and underfill compounds having the desired properties for thermal management applications in the electronics industry.
It is suggested in a literature article entitled xe2x80x9cBoron Nitride Powders Formed By Aerosol Decomposition of Poly(borazinylamine) Solutionsxe2x80x9d authored by Robert Paine et al (J. Am. Ceram. Soc. 74 (12) 3126-28 in 1991), that spherically shaped BN particles may be formed by reacting trichloroborazine with an appropriate aminosilane. The poly(borazinylanine) product is subsequently dissolved in liquid ammonia and injected as an aerosol into a reaction furnace at high temperature. This produces a powdered boron nitride product having an amorphous structure. Thereafter, by calcining (heat-treating) the BN powder in a nitrogen atmosphere at a temperature of 1600xc2x0 C., over a time period of at least 8 hours, hexagonal BN particles of overall spheroidal shape with protruding non-uniform crystallized blades can be formed. This process is not commercially viable since poly(borazinylamine) is very expensive and at present not yet available commercially. Moreover, the post-heat treatment operation does not produce uniform spherically shaped particles.
Another method for forming spherically shaped BN particles is taught in European Patent Publication No. 0 396 448 published on Nov. 7, 1990. Its basis is the reaction of boron trichloride and gaseous ammonia at high temperature followed by calcination at high temperature in a graphite furnace under vacuum. This method is unattractive because BCl3 is not only an expensive raw material but is also very reactive generating hydrogen chloride gas HCl(g), a corrosive, hazardous, byproduct. In addition, Cl is also an undesirable contaminant for use in thermal management applications.
A method has been discovered in accordance with the present invention for economically forming spherically shaped precursor BNxOy particles and for converting them to spherically shaped BN particles having either a highly crystalline hexagonal structure or a turbostratic structure of lower crystallinity. The process provides a substantial degree of control over the crystallinity of the product as well as over the size of the particles and the BN crystallites that make up the particles within a given size range. Moreover, the concentration of oxygen in the precursor product can be accurately controlled.
The process of the present invention broadly comprises the steps of adding a boron compound to an aqueous solution, creating an aerosol spray from the boron containing aqueous solution in the form of aerosol droplets, injecting the aerosol droplets with an inert carrier gas such as N2 or Ar into a furnace at preset flow rate, maintaining the furnace temperature within a temperature range of between 600xc2x0 C. and 1500xc2x0 C., preferably 1000xc2x0-1200xc2x0 C., and injecting a gaseous nitriding agent into the furnace simultaneously with the injection of the aerosol droplets with the flow rate of the gaseous nitriding agent maintained in a direct proportion to the flow rate of the aerosol such that the ratio of flow of the gaseous nitriding agent to said preset flow rate for the carrier gas and aerosol droplets whereby spherically shaped BNxOy precursor particles are formed and subsequently heat treating the BNxOy precursor particles into particles of spherically shaped BN having either a turbostratic or a hexagonal structure.
The system for forming spherical BN in accordance with the present invention comprises: an aerosol generator for forming an aerosol spray consisting of droplets of an aqueous boron containing compound; a means for injecting a gaseous N2 carrier including said aerosol droplets into a heated furnace at a controlled flow rate, and a means for simultaneously injecting into the furnace a gaseous nitriding agent.
It has been further discovered in accordance with the present invention that the precursor BN product produced before calcining consists of mainly amorphous spherical particles with a complex BNxOy composition and under preferred conditions having a controlled oxygen concentration of between about 10 and 35%. Under other conditions higher oxygen concentrations may be realized.